1. Field of the Invention
The present invention relates to electronics, and, in particular, to the packaging and configuration of electronic components, such as high-power amplifiers, into electronic modules.
2. Description of the Related Art
The cellular and personal communication systems market continues to move toward second-and third-generation wireless interfaces such as generalized packet radio service (GPRS), CDMA2000, and wideband CDMA (WCDMA). To meet the needs of these markets, radio-frequency (e.g., 400 MHz to 3 GHz) high-power (e.g., 30 W to 300 W or more) amplifiers (HPAs) are being designed to be smaller, more efficient, lower cost, and easier to manufacture. The power gain, output power, efficiency, and linearity of HPAs are key parameters that affect the overall performance and cost effectiveness of these communications systems.
A number of factors present challenges to the designers of HPA circuits. These include thermal management, the control of quiescent currents and package parasitics, compensating for non-linear characteristics of gain as a function of input power, temperature, and/or bias, and impedance matching.
In order to avoid permanent damage to the electronic component itself as well as to surrounding elements, an electronic component that generates a relatively large amount of heat, such as a power amplifier, is typically assembled in a package that has one or more heat sinks that help dissipate the heat generated during operation of the electronic component.
FIGS. 1A and 1B show isometric and cross-sectional views of a prior-art HPA package 100. HPA package 100 houses a high-power amplifier implemented using six integrated circuit (IC) dies: two input dies 102, two amplifier dies 104, and two output dies 106, where dies 102a, 104a, and 106a are electrically interconnected in series via wire bonds 108, dies 102b, 104b, and 106b are electrically interconnected in series via other wire bonds 108, and each die may include thousands of integrated devices. For example, each amplifier die 104 typically includes thousands of transistors configured in parallel. Input dies 102 are also electrically connected to a single input conductor 110 via wire bonds, and output dies 106 are also electrically connected to a single output conductor 112 via wire bonds.
In operation, a single, low-power electrical input signal is applied to HPA package 100 at input conductor 110, which passes the input signal to input dies 102. Input dies 102 condition the input signal and apply the conditioned input signals to amplifier dies 104. Amplifier dies 104 amplify the conditioned input signals and apply the resulting high-power signals to output dies 106. Output dies 106 condition the high-power signals and apply the conditioned, high-power signals to output conductor 112, which presents the conditioned, high-power signals as a single high-power electrical output signal from HPA package 100.
During operation, dies 102–106 generate a relatively large amount of heat that needs to be removed from the dies and dissipated in order avoid damaging elements within HPA package 100 as well as other components near HPA package 100. To achieve this thermal control, dies 102–106 are mounted directly to a metal carrier 114, e.g., using an electrically and thermally conducting solder material, to form a die/carrier assembly. In addition to functioning as a heat sink that conducts heat away from the dies, the electrically conducting metal carrier 114 also functions as a base conductor for HPA package 100 that provides the substrate voltage (e.g., ground or drain voltage) for the dies.
This die/carrier assembly is itself mounted within a package body 116 to form an HPA package that is then solder-mounted onto a metal chassis having heat-radiating fins (not shown). In order to withstand the temperatures associated with the solder-mounting of the HPA package onto the metal chassis, package body 116 is typically made from a heat-resistant ceramic material or a high-temperature liquid crystal polymer material. In order to avoid damage resulting from different thermal expansion properties, carrier 114 is typically made from a metal, such as a copper tungsten alloy, whose coefficient of expansion substantially matches that of the package body's ceramic material.
Although the design of HPA package 100 provides thermal management for the HPA circuitry, it does not address other design goals for high-power amplifiers such as the control of quiescent currents and package parasitics, compensating for non-linear characteristics of gain as a function of input power, temperature, and/or bias, and impedance matching. In order to address these goals, HPA package 100 is typically mounted on a circuit board (CB) that contains other electrical components designed to provide these functions.
FIG. 2 shows a block diagram of a prior-art HPA system 200 that might be implemented on a single circuit board. HPA system 200 has a number of amplifier stages 202 connected in series via impedance-matching (Z-match) circuitry 204, which provides impedance matching between the amplifier stages. Each amplifier stage 202 includes an HPA package 206 and sensor & bias circuitry (SBC) 208, which provides such functions as control of quiescent currents and package parasitics and/or compensating for non-linear characteristics of gain as a function of input power, temperature, and/or bias. Each HPA package 206 in HPA system 200 may be an instance of HPA package 100 of FIGS. 1A and 1B. The operations of each SBC 208 are controlled by microcontroller 210, which uses the data-storage capabilities of non-volatile memory 212. Input and output Z-match circuitry 214 and 216 provide impedance matching between HPA system 200 and external electrical components.
In operation, a low-power RF input signal 218 is applied to HPA system 200 at input Z-match circuitry 214. Depending on the particular implementation, the input signal may be passed to the first HPA package 206 either directly or via its SBC 208. Similarly, depending on the particular implementation, the amplified signal generated by HPA package 206 may be passed to the first Z-match circuitry 204 either directed or via its SBC 208. Analogous pre-and/or post-amplifier processing may also be applied by each SBC 208 at each amplifier stage 202. Each amplifier stage 202 increases the amplification of the input signal until a resulting high-power RF output signal 220 appears at output Z-match circuitry 216.
Each SBC 208 includes elements such as a temperature sensor, a drain current monitor, analog-to-digital converters (A/Ds), digital-to-analog converters (D/As), and bias circuits. The SBC helps manage the temporal and thermal compensation of the high-power amplifiers and the configuration of each amplifier in one of a number of different possible operating modes (e.g., inverting amplifier, unity follower, buffer, and non-inverting preamplifier).
Microcontroller 210 receives amplifier status information (e.g., package temperature and RMS drain current measurements) from each SBC 208 and generates and sends control information (e.g., gain and/or compensation changes) to each SBC 208.
HPAs provide from 5 W to more than 300 W of output power per channel, and, for applications such as cellular base stations, HPAs often require extremely good linearity to maximize the data throughput in a given channel. One consideration in achieving linearity in these applications is the DC biasing of the transistors from which the HPAs are constructed. For metal-oxide-semiconductor, field-effect-transistor (MOSFET)-based amplifiers, for example, the quiescent drain current of the MOSFET should be held substantially constant over temperature and time for optimal performance. Typically, the target accuracy for drain-current stability over temperature is ±5%; however, ±1% is more desirable for a high-performance, wideband design. Drain-current drift in a typical amplifier will result in reduced power output, increased distortion products, and reduced phase linearity, all of which impair the performance in digital communications systems. Thus, it is important to monitor this parameter.
It is common to employ laterally-diffused metal-oxide-semiconductor (LDMOS) transistors for HPA designs. The quiescent drain current on these transistors, as an example, can be set by adjusting the gate-to-source voltage and monitoring the drain current. Ideally, drain current would be constant over temperature. However, since the gate threshold voltage (which is a component of the gate-to-source voltage) of an LDMOS device varies with temperature, some type of temperature compensation is typically used to maintain constant drain current. For example, to bias an LDMOS device #21090 from Agere Systems, of Allentown, Pa., as a class AB amplifier, the gate-to-source voltage is nominally set to 3.8 volts at 35 degrees Celsius to obtain a quiescent drain current of 400 mA. However, for every five degrees Celsius increase in junction temperature of the LDMOS device, the gate-to-source voltage should be reduced by about 0.2 volts in order to maintain the drain current at 400 mA. Typically, the drain current is first monitored during device setup when no RF input is applied to the HPA. The slope of the gate-to-source voltage versus temperature is typically constant over a normal range of operating temperature, but the intercept differs from wafer to wafer and device to device. Thus, device-specific parameters are typically stored for each HPA package, usually in a non-volatile memory, such as memory 212.
Temperature compensation is usually accomplished by table lookup or on-the-fly calculation based on stored characteristics and known drain current, gate voltage, and temperature relationships. Compensating for the temperature of LDMOS devices can be problematic, however, because of the difficulty of obtaining accurate and timely LDMOS junction-temperature information.
In exemplary prior-art HPA system 200 of FIG. 2, microcontroller 210 receives digitally sampled temperature information from each SBC 208 and adjusts the drain current of each HPA package by changing the amplifier's gate-to-source voltage based on values stored in a look-up table (LUT) in memory 212. Memory 212 contains a LUT for each HPA package 206 in HPA system 200. For each HPA package, the microcontroller uses the temperature information from the local SBC as a look-up reference into the LUT for that HPA package. The contents from the corresponding memory location are read by the microcontroller from the memory and then loaded by the microcontroller into a digital-to-analog converter (DAC) internal to the SBC associated with the HPA package. The output of the DAC determines the gate-to-source voltage of the HPA and can be used to keep the drain current relatively constant over temperature.
In other embodiments of the prior art, the SBC may alternatively or additionally monitor quiescent and total current directly. One of the disadvantages of the design of HPA system 200 is that the temperature sensor (e.g., a thermocouple) in the SBC is located external to the (typically ceramic) package body of the HPA package. Variations in the thermal-transfer characteristics of the HPA package can affect the accuracy of temperature compensation. Delays between when temperature changes occur at the transistor junction and when such changes are indicated external to the HPA package can also affect the ability of the temperature-compensation circuit to manage stabilization of drain current as a function of temperature when large changes in drain current are desired.
In conventional implementations, impedance-matching circuits 204, 214, and 216 need to be tuned to take into account variations resulting from different distances between adjacent HPA packages 206 both within a single HPA system 200 and in different instances of HPA system 200, as well as different characteristics of the individual HPA packages, which vary from wafer to wafer and from device to device. In the event of a failure of one of the HPA packages, it is insufficient to simply replace the failed package with a new one. This is because each HPA package has unique input and output impedances and unique passband characteristics. For this reason, HPA system 200 typically includes tunable impedance-matching circuits. Even so, if a single package on a circuit board fails, it is typically cheaper and easier to replace the entire board rather than try to adjust the board to accommodate a replacement package. These factors result in inefficiency, waste, and high cost of systems designed with these packages.